1. Field of the Invention
The present invention relates to a digital indicator for measuring a dimension and the like of a to-be-measured article and a method for using the digital indicator.
2. Description of Related Art
Indicators provided with a spindle have been used as a measuring device for measuring a dimension and the like of a to-be-measured article.
The general indicators include pointer indicators that display a displacement amount of the spindle by a rotation angle of the pointer as well as digital indicators that electrically detect and display a displacement amount of the spindle.
Such a digital indicator is provided with a main body, a spindle penetrating an outer circumferential wall of the main body and supported to be slidable in an axial direction thereof, an encoder (a detector) for detecting a sliding amount of the spindle and a display digitally displaying a displacement amount of the spindle relative to the main body.
For example, JP-A-2003-344004 (the second embodiment) discloses a digital indicator in which a main body and a bearing that is provided on the main body and slidably supports a spindle are integrally molded with conductive synthetic resin in order to protect an electric circuit from magnetism outside.
The detector used in the digital indicators may be an electrostatic-capacitance, electromagnetic or optical encoder. For instance, the detector may be an encoder including an electrostatic-capacitance scale provided along an axial direction of the spindle and a detector head that is provided on the main body and is capacitively-coupled with the scale to detect a displacement amount of the spindle. Such a conventional encoder measures a current displacement amount by incrementing a displacement from a reference point of the scale.
FIG. 2(A) schematically shows a positional relationship between the scale and the detector head of the digital indicator.
As shown in FIG. 2(A), a scale 15 opposes to a detector head 16 that detects the displacement of the scale 15. The displacement of the spindle can be acquired by incrementing the detected displacement of the scale 15.
However, as shown in FIG. 2(B), when the spindle is excessively slid upward, the scale 15 goes beyond the detector head 16. Hence, the detector head 16 cannot detect the displacement of the scale 15, which causes a failure of the detector, so that the displacement of the spindle cannot be obtained. Accordingly, the conventional digital indicator is arranged to have a slidable range of the spindle in which the scale 15 cannot go beyond the detector head 16 i.e. a slidable range equal to a detectable range of the encoder. Note that “a” shown in FIG. 2(A) indicates the slidable range of the spindle and the detectable range of the encoder.
The conventional digital indicator has a following problem.
FIGS. 3 and 4 show a measurement of a thickness of a bottom of a glass 2 using the digital indicator.
To measure the bottom thickness of the glass 2, initially a contact portion 12 of a spindle 11 is brought into abutment on a floor surface 3 to conduct zero-setting. Next, as shown in FIG. 3, the spindle 11 is slid upward until the contact portion 12 is positioned above the height of a wall of the glass 2. While the spindle 11 is maintained at the position, the glass 2 is horizontally moved until the bottom of the glass 2 is positioned below the spindle 11. As shown in FIG. 4, the spindle 11 is then slid downward until the contact portion 12 abuts on the bottom surface of the glass 2, whereby obtaining the thickness of the bottom of the glass 2 from a value displayed on a display 14.
Although the slidable range of the spindle 11 needs to be larger than the height of the wall of the glass 2 in such a measurement, the slidable range of the spindle in the conventional digital indicator is equal to the detectable range of the encoder. Hence, even when the length of a to-be-measured portion (the bottom of the glass 2) is small, a costly and bulky indicator having a large detectable range is required.